The present invention relates generally to hydraulic drive systems for vehicles.
In certain vehicles, a hydraulic system is used to move the vehicle. When the vehicle turns left or right, the turning wheels rotate at different speeds because of the turning radius of the vehicle. If the hydraulic system is connected to provide power to the turning wheels, the system must compensate for this differential, to maintain power and traction to the vehicle. The system must also compensate for different loads, and whether the vehicle is on an upgrade or downgrade, as well as when one wheel loses traction.
A number of hydraulic drive systems have been developed in an attempt to power the vehicle under different loads and when the vehicle is turning.
Some systems use motors connected in parallel, that is, a single input is provided which is then split off to the right and left motors. Flow divider/combiner units or other types of control valves are used to control the flow rates to the motors, in proportion to the steer angle. See, e.g., U.S. Pat. Nos. 3,149,464; 3,978,937; and 5,181,579. While such systems may be appropriate for certain applications, the parallel plumbing of the systems adds complexity and cost to the hydraulic drive system. In certain applications, the space available for the hydraulic circuit and motors is at a premium, and it is incumbent to keep the components as compact as possible. It is believed at least some of such prior devices fail in this respect.
It is therefore believed there is a demand in the industry for a hydraulic drive system for a vehicle which compensates for the load and direction of the vehicle, so that power is maintained to the vehicle in different directions, under different loads, and even when a drive wheel loses traction, and where the system is simple, efficient and compact.